In recent years, secondary batteries have been increasingly developed as storage devices used for personal computers, mobile terminal devices, such as mobile phones, video cameras, satellites, vehicles, and the like, to make the devices ultrathin and compact. As packaging materials used for such batteries, laminated packaging materials formed of a multilayer film (e.g., with a configuration of base material layer/first adhesive layer/metal foil layer/second adhesive layer/heat-sealable resin layer) are attracting attention. Unlike metal cans that have conventionally been used as containers, the laminated packaging materials formed of a multilayer film are superior to the metal cans from the viewpoint that the laminated packaging materials have light weight and high heat dissipation properties, and allow for free selection of shape.
Power supply from a secondary battery provided with such a packaging material of a multilayer film involves use of metal terminal members, referred to as tabs, which are respectively connected to a positive electrode and a negative electrode. The tabs each include a metal terminal (lead) and a resin film (tab sealant) covering the metal terminal. For example, in a lithium ion secondary battery, aluminum is typically used for the positive-electrode lead, and copper for the negative-electrode lead. The tab sealant is interposed between each lead and the packaging material, and required to satisfy the following performances.
In the first place, a tab sealant can have adhesiveness to both a lead and a packaging material. Adhesion between a packaging material and a tab sealant can be ensured by the tab sealant being made of a heat-sealable resin. Adhesion between a lead and a tab sealant can be improved by acid-modifying a polyolefin resin used for the tab sealant. Furthermore, when a tab sealant is sealed to a lead, a lead end portion can be filled with a tab sealant without forming a gap. When the filling is insufficient, a gap is formed between the tab sealant and the lead, causing leakage of contents or peeling.
In the second place, a tab sealant can ensure insulation between a lead and a packaging material. When a tab sealant and a lead are heat sealed, there is a concern that the thickness of the tab sealant is reduced, depending on the pressure or temperature conditions, and insulation is not ensured.
In particular, an end portion of a lead is most likely to be reduced in thickness. Measures required to be taken against the thickness reduction include reducing a melt flow rate to make the resin less fluid, or using a resin having a high melting point to make the resin less meltable, in a portion of the tab sealant.
Components included in a secondary battery contain substances that react with water infiltrating into the battery and can cause deterioration of the battery performance or corrosion of the members configuring the secondary battery. When a lithium ion secondary battery is taken as an example, it includes a lithium salt, such as lithium hexafluorophosphate or lithium tetrafluoroborate, as an electrolyte. When such a lithium salt reacts with water, hydrofluoric acid is generated, possibly causing corrosion and deterioration of the battery. Therefore, the laminated packaging material is configured to include a metal foil layer to prevent infiltration of moisture into the secondary battery through the surface of the packaging material.
For tabs, some methods are also reported as preventing infiltration of moisture through a tab sealant. For example, a tab sealant of PTL 1 has a configuration in which polyolefin layers are provided to both surfaces of a polyethylene naphthalate film. With this configuration of using a polyethylene naphthalate film having higher moisture barrier properties than polyethylene terephthalate, moisture infiltration is better prevented while trying to improve electrical insulation.